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Search for Satellites Near Comet 67P/Churyumov-Gerasimenko Using Rosetta/OSIRIS Images

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Search for Satellites Near Comet 67P/Churyumov-Gerasimenko Using Rosetta/OSIRIS Images A&A 583, A19 (2015) Astronomy DOI: 10.1051/0004-6361/201525979 & c ESO 2015 Astrophysics Rosetta mission results pre-perihelion Special feature Search for satellites near comet 67P/Churyumov-Gerasimenko using Rosetta/OSIRIS images I. Bertini1, P. J. Gutiérrez2, L. M. Lara2, F. Marzari3, F. Moreno2, M. Pajola1, F. La Forgia3, H. Sierks4, C. Barbieri3, P. Lamy5, R. Rodrigo6;7, D. Koschny8, H. Rickman9;10, H. U. Keller11, J. Agarwal4, M. F. A’Hearn12, M. A. Barucci13, J.-L. Bertaux14, G. Cremonese15, V. Da Deppo16, B. Davidsson9, S. Debei17, M. De Cecco18, F. Ferri1, S. Fornasier13;19, M. Fulle20, L. Giacomini21, O. Groussin22, C. Güttler4, S. F. Hviid23, W.-H. Ip24;25, L. Jorda22, J. Knollenberg23, J. R. Kramm4, E. Kührt23, M. Küppers26, M. Lazzarin3, J. J. Lopez Moreno2, S. Magrin3, M. Massironi21, H. Michalik27, S. Mottola23, G. Naletto28;16;1, N. Oklay4, N. Thomas29, C. Tubiana4, and J.-B. Vincent4 (Affiliations can be found after the references) Received 27 February 2015 / Accepted 11 May 2015 ABSTRACT Context. The European Space Agency Rosetta mission reached and started escorting its main target, the Jupiter-family comet 67P/Churyumov- Gerasimenko, at the beginning of August 2014. Within the context of solar system small bodies, satellite searches from approaching spacecraft were extensively used in the past to study the nature of the visited bodies and their collisional environment. Aims. During the approaching phase to the comet in July 2014, the OSIRIS instrument onboard Rosetta performed a campaign aimed at detecting objects in the vicinity of the comet nucleus and at measuring these objects’ possible bound orbits. In addition to the scientific purpose, the search also focused on spacecraft security to avoid hazardous material in the comet’s environment. Methods. Images in the red spectral domain were acquired with the OSIRIS Narrow Angle Camera, when the spacecraft was at a distance between 5785 km and 5463 km to the comet, following an observational strategy tailored to maximize the scientific outcome. From the acquired images, sources were extracted and displayed to search for plausible displacements of all sources from image to image. After stars were identified, the remaining sources were thoroughly analyzed. To place constraints on the expected displacements of a potential satellite, we performed Monte Carlo simulations on the apparent motion of potential satellites within the Hill sphere. Results. We found no unambiguous detections of objects larger than ∼6 m within ∼20 km and larger than ∼1 m between ∼20 km and ∼110 km from the nucleus, using images with an exposure time of 0.14 s and 1.36 s, respectively. Our conclusions are consistent with independent works on dust grains in the comet coma and on boulders counting on the nucleus surface. Moreover, our analysis shows that the comet outburst detected at the end of April 2014 was not strong enough to eject large objects and to place them into a stable orbit around the nucleus. Our findings underline that it is highly unlikely that large objects survive for a long time around cometary nuclei. Key words. comets: general – comets: individual: 67P/Churyumov-Gerasimenko – planets and satellites: detection – techniques: photometric 1. Introduction to be composed of objects with an orbit intermediate between TNOs and short-period comets (Levison & Duncan 1997). The detection and study of small-body satellites is an important Spacecraft encounters allow satellite searches and discov- tool for investigating the nature, origin, and evolution of aster- eries down to sizes much smaller than possible from Earth, oids and comets. Measuring the orbit of small companions al- which also adds the advantage of effectively investigating the lows determining the mass of the system and of the primary. space closer to the objects. Several satellite searches were per- From this, its bulk density is derived when the volume is known. formed using data from NASA, JAXA, and ESA missions. We This provides hints on the physical composition of the object mention the studies of NASA/Galileo at (951) Gaspra (Belton and its internal structure. Studying the connected systems also et al. 1992) and (243) Ida (Belton et al. 1995), NASA/NEAR at provides clues on the collisional events that occurred during the (253) Mathilde (Veverka et al. 1999) and (433) Eros (Veverka early stages of the formation of the solar system and its subse- et al. 2000), JAXA/Hayabusa at (25143) Itokawa (Fuse et al. quent evolution (Merline et al. 2002). 2008), ESA/Rosetta at (21) Lutetia (Bertini et al. 2012), and At the time of writing (beginning of May 2015), we know finally NASA/Dawn at (4) Vesta (Memarsadeghi et al. 2013). of 256 small bodies that have companions of different sizes1. Except for the encounter with Ida, which provided the first di- Among them there are 55 near-Earth asteroids (NEAs), 20 Mars- rect and definitive evidence of the existence of asteroid com- crossers, 97 main belt asteroids (MBAs), 4 Jupiter Trojans, and panions with the serendipitous discovery of the small moon 80 trans-Neptunian objects (TNOs). Three TNOs that display Dactyl in 1993, all other searches were unsuccessful in detect- complex systems belong to the Centaur class, which are assumed ing small companions. These studies allowed placing important constraints on the size limit of possible satellites, however, pro- viding hints on the collisional history of the primary bodies. 1 http://www.johnstonsarchive.net/astro/asteroidmoons. A double nucleus with two possibly bound components was html claimed to explain the photometric anisotropies in the inner Article published by EDP Sciences A19, page 1 of8 A&A 583, A19 (2015) coma of the large comet C/1995 O1 Hale-Bopp based on both Short-exposure frames were taken for their relevance if a large ground-based (Marchis et al. 1999) and HST (Sekanina 1997) satellite had been detected, since they would have provided un- data. However, as underlined in Noll et al.(2006), Weaver & saturated views of the object. The NAC broadband orange filter Lamy(1997) reported no evidence of the second companion us- (with a central wavelength and FWHM of 649.2 nm and 84.5 nm, ing the same HST dataset, showing that no final univocal con- respectively) in the visible red domain was chosen to provide clusion on the binary nature of the system could be derived. the best S/N for possible satellites within a fixed exposure time. Decimeter-sized icy particles were found in the close vicinity Considering the image scale, the observed field of view (FoV) of the nucleus of the hyperactive comet 103P/Hartley2 during covered the inner ∼110 km from the comet optocenter. When the flyby of the NASA/EPOXI spacecraft performed on 2010 we mention a distance, it refers to the “projected distance”. November 4 (A’Hearn et al. 2011; Kelley et al. 2013; Hermalyn Assuming a Hill sphere radius of ∼650 km derived from the et al. 2013). Moreover, several cometary nuclei visited by space measurement of the comet mass by the Rosetta Radio Science missions (e.g., 1P/Halley, 19P/Borrelly, 103P/Hartley 2, and Investigation (RSI) instrument, namely 1:0 × 1013 kg (Sierks 67P/Churyumov-Gerasimenko itself) showed complex irregular et al. 2015, and references therein), and from the heliocentric shapes that can be interpreted as the results of the evolution distance-dependent formula in Hamilton & Burns(1991), we of contact binary systems. Despite these interesting consider- note that our FoV intersected a three-dimensional space corre- ations, no classical satellite searches have been performed for sponding to ∼37% of the comet gravitational sphere of influence. comets, as was extensively done for asteroids, and no solid ma- The first and the last images obtained for the satellite search terial larger than ∼1 m orbiting a comet has ever been unambigu- are shown in Fig.1. ously discovered. During the approach to comet 67P/Churyumov- Gerasimenko in July 2014, before the orbit insertion performed 3. Analyzing the data at the beginning of August 2014, the two-camera instrument OSIRIS (Keller et al. 2007) onboard Rosetta took several When we analyzed the data, we first estimated the expected images with the purpose of detecting and studying possible results for a potential satellite by performing a Monte Carlo objects orbiting the comet in the vicinity of the nucleus. The simulation. To do this, we selected 50 000 clones randomly lo- search had the additional aim to ensure spacecraft safety. The cated inside the Hill sphere, with a random velocity with a discovery of solid blocks close to the nucleus would have smaller modulus than the escape velocity. We calculated the implied that special care was necessary so that the spacecraft clone positions within the CCD sensor reference frame for the trajectory would not cross any orbiting material. An appropriate time in which the long-exposure images were taken. A value observational strategy was defined so that the data acquisition of 1.0 m s−1 was used for the escape velocity, in accordance and reduction processes were optimized, maximizing the pos- with Sierks et al.(2015). In a first approach, acceleration e ffects sibility of detecting and measuring the orbital arc of a possible on the potential satellite were neglected. The spacecraft posi- small companion. We here present the adopted observational tion and frame orientations were derived using appropriate spice strategy and the data analysis, together with the results of our kernels. investigation. After calculating the clone positions, we measured their dis- placements for three different cases: (1) within a single exposure time to determine whether the potential satellite showed a track 2. Observational strategy in a single frame; (2) between individual images to determine It is well known that satellite searches from spacecraft images whether it was possible to apply median averaging of images are affected by several problems such as the fast motion of the within the same run in order to eliminate possible cosmic rays camera with respect to the target and the bona-fide detection of and spurious signals; (3) between runs of images to limit the ra- interesting point-like objects against background stars, cosmic- dius search for displacements of the potential satellite.
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